The device::iommu_group field may be set even if no IOMMU is in use.
iommu_present() is still a better indicator, although it doesn't tell
us whether *our* device is affected.
Reported-by: NAlex Williamson <alex.williamson@redhat.com>
Signed-off-by: NBen Hutchings <bhutchings@solarflare.com>